This invention relates to a semiconductor thermally sensitive switch structure utilizing a thyristor.
In order to switch thyristors from their OFF to their ON state, there have been previously employed various means. For example, (1) an overvoltage or an OFF-state voltage having an excessive rate of rise has been applied across the main electrode of thyristors as in semiconductor diode switches, semiconductor symmetrical switches etc.; (2) a gate trigger current has been supplied to the gate electrode of thyristors as in the usual triode thyristors; (3) semiconductor pellets of thyristors have been irradiated with light as in photosensitive thyristors; (4) semiconductor pellets have been pressurized to increase the leakage current resulting from the strain thereon, thereby to turn the thyristors on as in pressure sensitive thyristors; and (5) thyristors have been put in an electric field to increase the surface conductance thereby to turn them on with an increased leakage current as in field effect thyristors. Among these means, the means (1), (2) and (3) have been most frequently employed for practical purposes.
What is common to those known manners is to use thyristors to maintain the junction temperature of the thyristor equal to or less than the prescribed magnitude thereof so as to prevent the OFF-state voltage ability or the breakover voltage and the circuit commutated turn-off time thereof from decreasing below the prescribed magnitudes thereof. In other words, if the junction temperature of thyristors exceeds the prescribed magnitude thereof then the breakover voltage is decreased so as to break the thyristors over with the particular applied voltage. This leads to a danger that thyristors and circuits in which they are connected have the excessively high current flowing therethrough resulting in destruction. Also, with the circuit commutated turn-off time exceeding the prescribed magnitude thereof, the commutation fails to be effected. This results in the flow of extraordinary current through associated circuits which is accompanied by the destruction or disablement of the thyristors and associated utilization devices.